5.5.1 Requests

A request line begins with a control character, which is either a single
quote (‘'’, the no-break control character) or a period
(‘.’, the normal control character). These can be changed;
see Character Translations, for details. After this there may be
optional tabs or spaces followed by an identifier, which is the name of
the request. This may be followed by any number of space-separated
arguments (no tabs here).

Since a control character followed by whitespace only is ignored, it is
common practice to use this feature for structuring the source code of
documents or macro packages.

.de foo
. tm This is foo.
..
.
.
.de bar
. tm This is bar.
..

Another possibility is to use the blank line macro request blm by
assigning an empty macro to it.